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Ortec Model 9327 Timing Discriminator
'Ortec Model 9327 Timing Discriminator: '''There are two of these, one for the electron MCP’s (multi-channel plates.) , and one for the ion MCP. PURPOSE OF THE 9327: The anode signal is a charge limited pulse that has lots of height variation. The charge limit means that the signal quickly gets smaller with increasing cable length. The variation in height is difficult for other pieces of data collection electronics to handle. The 9327 solves both of these problems by converting this fragile signal, first into a pre-amplified signal that is not charge-limited, and then into a robust TTL signal of constant (4.5 volt) height and (100ns) width. The amplified signal is available through the AMP OUT sma connector. The TTL signal is what is used by all of our data collection electronics. The signal can be transmitted through the long cables with no loss of fidelity. In addition, it has very little (sub-nanosecond) jitter with respect to the original pulse. Finally, although the preamp is not cheap (close to $1,000) it is much cheaper and easier to replace that the data electronics downstream from it. The anode signal can sometimes experience high-voltage gliches. This preamp also serves as a first line of defense, protecting other electronics from such gliches. NOTE: The ortec MCS card STOP and START pulses should be 2.5 volt pulses. Thus it is not wise to plug the 9327 directly into the 9353. It will not damage the card, but it will screw up its resolution. The 9327 output should go either into the ''voltage to random frequency converter, or directly into the fast trigger box. POWER SUPPLY FOR THE 9327: The 9327 requires a power supply. This supply is in a blue box that also has a bunch of knobs, whistles, and timing electronics. These knobs and timing electronics are no longer used, having been made obsolete by the fast sync box. The blue box is now only a power supply. HOW TO SET THE 9327 THRESHOLD: The 9327 has a screw-driver adjustable “Threshold” that must be set correctly before running an experiment. To set it, follow these steps: 1. DOUBLE CHECK: Before turning on any high voltage, make sure that all anode signals are terminated to 50 ohms! 2. OBSERVATION OF STRAY IONS DIRECTLY FROM THE ANODE: If you suspect the preamps might be blown, follow the next steps. Otherwise go directly to 3. a. Attach a BNC cable long enough to reach the scope directly to the anode output of the chamber. Terminate the end of the BNC with a 50 ohm BNC feedthrough.. Do this so you do not have to use the internal 50 ohm coupling on the scope, which is very fragile. If the MCP detector should have a high voltage spark, it could take down the scope. The other advantage of using the external 50 ohm load is you can play with the cabling without ever loosin g termination. It is very important that whenever high voltage is on the MCP plates, the anode is terminated to a 50 ohm load. b. With the anode signal attached to the scope through the 50 ohm feed through, set the time base to about a 10 ns per division and the sensitivity to about 5 mV per division. Set the input impedance to high Z (you are using an external load) and the signal DC coupled. Set the trigger to the scope channel you are using. c. Slowly adjust the trigger level from 0 volts to a negative level. For a trigger level of about -1mV, you should see a very high frequency signal that is not stray ions, but rather due to some odd lab noise. This signal comes in at the MHz level (limited by the scope to something more like 200Hz) and has very little pulse height variation. As you move the trigger level below about -3mV, you should start seeing stray ions. Stray ions are marked by a large variation in pulse height and relatively low frequency (0.1 to 100Hz, depending on the level of vacuum and the condition of the plates.) You may have to adjust the MCP voltages or get new MCP plates if healthy (5-50mV) stray ion signals do not occur. 3. OBSERVATION OF STRAY IONS AND SETTING OF THRESHOLD LEVEL a. If you have followed STEP 2, make sure to turn off the MCP voltage before changing the wiring. b. Remove any 50 ohm feedthrough that might be left on the BNC/sma cabling. Wire the anode directly into the input of the 9327 preamp, which will now provide termination for the anode signal. c. Take the AMP output of the 9327 and attach it to a scope. Set the time base to about a 5 ns per division and the sensitivity to about 5 mV per division. Set the input impedance to 50 ohms and the signal DC coupled. Set the trigger to the scope channel you are using. d. Slowly adjust the trigger level from 0 volts to a negative level. For a trigger level of about -1mV, you should see a very high frequency signal that is not stray ions, but rather due to some odd lab noise. This signal comes in at the MHz level (limited by the scope to something more like 200Hz) and has very little pulse variation. As you move the trigger level below about -3mV, you should start seeing stray ions. Stray ions are marked by a large variation in pulse height and relatively low frequency (0.1 to 100Hz, depending on the level of vacuum and the condition of the plates.) You may have to adjust the MCP voltages or get new MCP plates if healthy (5-50mV) stray ion signals do not occur. e. Adjust the trigger level of the scope so that the very fast noise signal is gone, but the maximum stray ion rate occurs. f. Now plug the TTL output of the 9327 into a second channel of the scope. Set the coupling to DC, 5 volts/division. Reset the time base to 20 ns /division. Continue triggering on the AMP output signal. g. Use a screwdriver to adjust the THRESHOLD of the 9327 until almost every stray ion that occurs is accompanied by a TTL pulse. The very smallest stray ions do not have to be accompanied by a pulse. h. Now set the trigger to a level of 2.5 volts on the TTL signal. Every TTL pulse observed should be accompanied by a stray ion. If the threshold for the TTL pulse is too low, some TTL pulses will be accompanied by garbage pulses or no pulses at the AMP out signal. If the threshold is too high, the count rate of TTL pulses will be much lower than the count rate of stray ions from the AMP output signal. i. Now take the TTL pulse into our Phillips frequency counter. Record the vacuum and count rate in the lab notebook you are using. j. Reconnect the TTL output to the data collection electronics.